The flux in any portion of the armature passes through p/2
c/r (cycles per revolution) or through (p/2 [(r/min)/60] Hz. The iron losses
consist of the hysteresis loss, which equals Kb1.6fw watts, and the eddy
current loss, which equals Ke(Bft)^2w watts.

K is the hysteresis constant of the iron used, Ke is a
constant inversely proportional to the electrical resistance of the iron, B is
the maximum flux density in lines per square inch, f is the frequency in hertz,
w is the weight in pounds, and t is the thickness of the core laminations in
inches.

The eddy loss is reduced by using iron with as high an
electrical resistance as is feasible. Very high resistance iron has a tendency
to have low flux permeability and to be mechanically brittle and expensive. It
is seldom justified in dc machines. The loss is kept to an acceptable value by
the use of thin core laminations, 0.017 to 0.025 in thickness.

Another significant loss is the pole-face loss. These losses
are included in the core loss.

Unfortunately, there are other losses in the core that may
differ widely even on duplicate machines and that do not lend themselves to
calculation. These include:

**1. Loss due to filing of slots.**When the laminations have been assembled, it will be found in some cases that the slots are rough and must be filed to avoid cutting the coil insulation. This burrs the laminations and tends to short circuit the interlaminar resistance.

**2. Losses in the solid spider, core end plates, and coil supports**from leakage fluxes may be appreciable.

**3. Losses due to nonuniform distribution of flux**in the rotor core are difficult to anticipate. In calculating core density, it is customary to assume uniform distribution over the core section.

However, flux takes the path of least resistance and crowds
behind the teeth until saturation forces it into the less used, longer paths
below. As a result of the concentration, the core loss, which is about
proportional to the square of the density, is greater than calculated.

Such test results are plotted in Fig. 8-54 for machines
using ordinary laminations 0.017 in thick and a limited amount of filing. They
do not include the pole-face losses, which would increase the values about 30%.

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